Demands for clean energy fuels such as hydrogen, methanol, liquid fuels (or gas-to-liquids (GTL)), or dimethyl ether (DME) are expected to grow significantly. Reforming equipment that are indispensable for manufacturing such synthetic gases are preferably large-sized ones with high thermal efficiency that are suitable for mass production. Further, reforming equipment for conventional petroleum refining or in petrochemical plants, for example, or ammonium production equipment or hydrogen production equipment using petroleum as a raw material, for example, are increasingly using heat exchange for waste heat collection to increase energy efficiency.
To effectively utilize such heat in high-temperature gases, heat exchange in the temperature range of 400 to 800° C., which is lower than conventional targets, is becoming important. Corrosion accompanying carburization of high-Cr-high-Ni—Fe alloy-based metal materials used in reaction tubes or heat exchangers in this temperature range is drawing attention as a problem.
Synthetic gases produced by such reaction equipment, that is, gases containing H2, CO, CO2, H2O and a hydrocarbon such as methane are usually in contact with the metal material of the reaction tube or the like at a temperature of 1000° C. or higher. In this temperature range, elements such as Cr or Si which have stronger tendencies to oxidize than Fe or Ni, for example, are selectively oxidized on the surface of the metal material such that a fine film of chromium oxide, silicon oxide or the like is formed, thereby preventing corrosion. However, in members at relatively low temperatures, such as heat-exchanging members, the diffusion of elements from the interior of the metal material toward the surface is insufficient such that the formation of oxidized films with corrosion-preventing effect is delayed. Further, since gases with a composition including hydrocarbon become carburizing, carbon enters the metal material through its surface, causing carburization.
When carburization progresses in an ethylene decomposition furnace tube, for example, and a carburization phase made of carbides of Cr or Fe, for example, is formed, this portion expands in volume. As a result, fine cracks can easily develop and, in the worst case scenario, the tube breaks while being used. Further, when a metal surface is exposed, carbon deposition (or caulking) occurs at the surface, with metal working as a catalyst, decreasing the flow passage area in the tube or decreasing heat transfer properties.
Also, heating furnace tubes in a catalytic cracking furnace which increase the octane number of naphtha produced by distillation of petroleum can be a severely carburizing environment made of hydrocarbon and hydrogen, causing carburization or metal dusting.
If such cracking, wear or clogging of the tube spread, this may cause a defect or the like in the equipment and interrupt its operation. In view of this, sufficient consideration is needed for selecting materials for equipment members.
To prevent corrosion caused by such carburization or metal dusting, various measures have been developed.
Traditionally, for such equipment members, high-Cr-high-Ni—Fe alloys have been used. For example, JP 2001-107196 A discloses a welded joint where a chemical composition is defined and the relationship between the Si, Cu or S content and the Nb, Ta, Ti and Zr contents and the relationship between the Ni, Co and Cu contents are limited to certain ranges. According to this document, this welded joint has good corrosion resistance and weld-crack resistance in a sulfuric-acid environment. However, this welded joint has a low Si content, making it difficult to use in an environment where metal dusting may occur.
JP 2002-235136 A discloses a welded joint made of an Ni-based heat-resistant alloy where it is proposed to actively include Al and a relationship between the amount of grain-boundary melting and the fixing force at grain boundaries is defined. According to this document, this welded joint has good carburization resistance and high-temperature strength. However, in this welded joint, an increase in the Si content to ensure metal-dusting resistance may cause solidification cracks to develop during welding, making it difficult to provide both metal-dusting resistance and solidification-cracking resistance during welding.
WO 2009/107585 proposes a metal material with increased C in a steel containing Si and Cu to reduce crack sensitivity in a heat-affected zone (hereinafter referred to as HAZ) during welding. However, a high C content increases solidification crack sensitivity during welding and also decreases creep ductility.
JP 2007-186727 A and JP 2007-186728 A propose including one or more of P, S, Sb and Bi in appropriate amounts to reduce gas dissociative absorption (i.e. gas/metal surface reaction). These elements segregate on a metal surface, which makes it possible to reduce carburization and corrosion due to metal dusting significantly even when they are not added in excessive amounts. However, these elements segregate not only on a metal surface but also along the boundaries of metal crystal grains, which leaves problems in hot workability and weldability.
WO 2012/524983 A proposes a metal material where the C content in a steel containing Si and Cu is limited to reduce solidification crack sensitivity and the Ti and Al contents are limited to reduce HAZ crack sensitivity. However, this document does not disclose welding materials required to weld metal materials to construct a structure.
JP 2006-45597 A proposes a welding material and a welded joint using it where an appropriate amount of Ti is added to reduce the adverse effects of Si.